A variety of prior art irradiance sensors is currently available in the electronics industry. Techniques for converting radiation to visible images are also known in which the absorbed radiant energy is converted to heat to change a measurable property of a sensing element, such as the resistance of a resistor or the position of a bi-material cantilever. The sensed radiation includes ultraviolet, visible, infra-red, or terahertz irradiation, depending on the spectral absorption of the sensor element.
An example of a prior art radiation sensor is the deflectable micro-electromechanical (MEM) cantilever bi-material device formed of a bi-material, anchored on an insulating substrate. The bi-material portion of the micro-cantilever device is formed of two different materials sharing a common surface and having different coefficients of thermal expansion (CTE). Examples of bi-material MEM micro-cantilever devices and methods for forming the same are disclosed in U.S. Pat. No. 5,844,238 issued to Sauer et al. and U.S. Pat. Nos. 6,140,646 and 6,420,706 issued to Lurie et al.
The prior art bi-material MEM micro-cantilever devices bend, or deflect, when irradiation is absorbed by an absorber element of the micro-cantilever and heats the bi-material section of the micro-cantilever, causing one of the bi-materials to expand at a greater rate than the other bi-material and resulting in deflection or bending of the micro-cantilever. The terms bend and deflect are hereinafter used interchangeably. An example of a prior art micro-cantilever is shown in FIGS. 1 and 2.
FIG. 1 is a cross sectional view showing a prior art micro-cantilever 10. As shown, the micro-cantilever 10 is suspended over the substrate 12. Area 14 forms the opening between the substrate 12 and the micro-cantilever 10. The micro-cantilever 10 includes a thermal isolation region 16, a bi-material region 18 and an absorbing region 20. Within the absorbing region 20, the absorber material 22 helps absorb radiation which is incident on the micro-cantilever structure. In the bi-material region 18, a first bi-material film 24 and a second bi-material 26 are present. Bi-material films 24 and 26 are contiguous and have different coefficients of thermal expansion.
When radiation is incident upon the MEM structure, the micro-cantilever 10 is heated and bends because the two bi-material films 24 and 26 expand at different rates. FIG. 1 shows micro-cantilever 10 in an original position when the micro-cantilever is not exposed to radiation.
FIG. 2 shows the micro-cantilever structure of FIG. 1 in a deflected or bent position; in this case, away from the substrate 12. The distance H indicates the deflection of the tip of the micro-cantilever, from the original position. A variety of apparatuses and methods is available in the art to provide a visual image having an intensity that varies according to the degree of the deflection H of the micro-cantilever structure 100. More generally, various apparatuses and methods are available for providing optical imaging having an intensity which is determined by the amount of radiation sensed by a radiation sensor.
When radiation is incident on a prior art micro-cantilever being used as an image sensor, it is desired to produce a visible image having an intensity which varies monotonically with the intensity of the incident radiation, over a wide dynamic range. As a micro-cantilever device bends in response to incident radiation, it approaches a physical limitation to its degree of bending. For example, if a micro-cantilever device is fabricated to bend freely downward in response to incident radiation, the physical limitation is reached when the micro-cantilever touches the substrate over which it is formed. For a micro-cantilever device chosen to bend upward in response to incident radiation, this too will reach a physical limitation point past which it can no longer bend as a bi-material. When this point of the physical limitation of bending is approached, the micro-cantilever device is more resistant to bending and therefore, less responsive to additional radiation. An increase in the amount of incident radiation does not cause the same extent of bending as when the micro-cantilever is in the rest position.
While a significantly higher dose of radiation forces the micro-cantilever to bend slightly more towards its physical limitation, the degree of bending is not proportional and the device response is not linear in this region. As a result, the linear range of the device is limited. The relationship between deflection H and the temperature of the micro-cantilever 10 is limited to the linear region.
Moreover, after the physical limitation point is reached, additional incident radiation does not cause any further bending which limits the dynamic range of the device. Since the intensity of an optical image ultimately produced is based on the degree of bending, prior art devices have a poor dynamic range and are limited linearity, results in producing an image having the same shortcomings.
Various methods for sensing the degree of free bending are available in the art. Examples include optically measuring the distance between the micro-cantilever and the substrate, measuring the movement of an optical beam reflected from the tilting cantilever surface, or electrically measuring the capacitance of a capacitor which includes an electrode formed in the substrate and another electrode formed in the micro-cantilever above the substrate. Various methods for producing a visible image having an intensity based upon the extent of bending are also known.
A method for forming a micro-cantilever device is disclosed in U.S. Pat. No. 5,844,238 issued to Saur which describes an infrared imager using room temperature capacitance sensors. The infrared imager includes an array of capacitance sensors that operate at room temperature and each infrared capacitance sensor includes a deflectable plate which bends in response to absorbed thermal radiation relative to a non-deflectable second plate. In one embodiment each infrared capacitance sensor is composed of a bi-material strip which changes the position of one plate of a sensing capacitor in response to temperature changes due to absorbed incident thermal radiation.
Other prior art includes various configurations of physical micro-cantilever or other MEM structures. For examples, FIGS. 3 and 4 show two embodiments of MEM structures which move in response to incident radiation. Each of the structures shown in FIGS. 3 and 4 include rigid thermal isolation arms 16, bendable bi-material arms 18 and an absorber area 20. It can be understood that, in addition to the exemplar structures shown in FIGS. 1-4, various other configurations for MEMs structures which bend or deflect in response to incident radiation are well known to those skilled in the art.
Improvements have been made to increase the dynamic range and linearity of the radiation sensors using bi-material MEMs structures available in the art, by providing a radiation sensor using nulling circuitry along with a micro-cantilever structure, as disclosed in U.S. Pat. No. 6,420,706 issued to Lurie which describes optical detectors using nulling for high linearity and large dynamic range. The natural bending of the bi-material is resisted by an electronic feedback signal to retain the absorber plate at a nulling position; thus the feedback signal is a measure of the bi-material temperature, and hence the irradiance. Examples taught for feedback signals include a piezoelectric element to add a commanded stress to the micro-cantilever, a heater element to add a commanded heat load to the micro-cantilever, or inducement of an attractive electrostatic charge between the micro-cantilever and the substrate. Within the limits of the effectiveness of the feedback signal to return the micro-cantilever to null position, the linear range may be extended.
By their very function, bolometers are very sensitive thermometers, measuring differences in absorber plate temperature of the order of hundredths of a degree. Moreover they possess a limited range of linear response to irradiative heating. Thus, present-art bolometers typically require addition of a thermoelectric refrigerator/heater to stabilize the base temperature of the device. Aside from its cost and bulk, the disadvantage is that power consumed by this thermoelectric refrigerator/heater is typically the single largest power drain in the imaging system.
All of the above mentioned methods depend on various forms of analog measurement, with the many associated sources of noise in the analog signal. Typically the noise sources in a cantilever bolometer include thermomechanical noise in the cantilever plate and the generally much larger electronic noise sources attendant on the analog nature of the measurement: 1/f noise, shot noise, Johnson or resistor noise, and amplifier noise. Typically the sum of these noise components has been five to twenty times greater than the noise in the background radiation itself.
Another limit of prior art configurations has been the degree to which the bi-material bends for a given absorbed irradiance. It is desirable that the bi-material bend maximally for a given irradiance. However, this desirable bending has been limited in prior art by the choices of bi-material materials and bi-material design such as the difference in thermal expansion between the expansive bi-materials, the high thermal conductivity of the thermal isolating leg, and the stiffness of the cantilever support legs. The responsiveness of conventional micro-cantilevers is typically of the order of 0.04 microns per degree C., with a maximum known MEMS bi-material responsiveness of 0.25μ/° C. and is limited by three issues.
First, the prior art choice of bi-material materials has been limited to aluminum or gold as the expansive material, and silicon nitride as the relatively non-expansive material.
Second, the bending has been limited by a necessity to achieve rapid thermal stabilization of the temperature of the absorber plate and bi-material within a fraction of a video frame period, so that the bi-material bending can be measured before the end of the frame. This has typically required a thermal time constant of less than 10 milliseconds for a 30 Hz video frame rate, and preferably 2.5 milliseconds for a 60 Hz frame rate. The time constant of the cantilever is inversely proportional to the thermal conductance of the insulator leg, necessitating the latter to be relatively large. Thus, for a given irradiance the maximal bi-material temperature which is proportional to the thermal time constant, is decreased. Thus the measurement signal is decreased and the frame rate is limited to a low value.
Third, a limit of prior teaching has been the mechanical resonance of the cantilever itself because the support leg, which includes the insulator leg plus bi-material, must be made stiff enough that the freely supported mass of the absorber plate has a resonant frequency higher than that of ambient acoustic fields, typically higher than 20,000 Hz. Were the mechanical resonance 2000 Hz, for example, a violin note or a whistle could destroy the signal or the micro-cantilever itself. Consequently the coated absorber mass must be made small to the limits of practicality and the bi-material and insulator legs must be made relatively stiff and unresponsive.
Yet another limit of prior art bolometers is their necessity to “blink” frequently with the display periodically going visibly black, using a mechanical shutter to remove the target irradiance from the array and zero out all the various readings of the bolometer sensors as they slowly drift in base temperature. This is occasioned by the limited irradiance range of the bolometer: essentially all of the dynamic range of the bolometer is required for constructing the final image.
An A/C coupled thermal imaging system has a signal that is centered around the “local average temperature” without an offset or pedestal which occurs independently across the array. Because of this, the device only has to deal with the smaller A/C signal and it therefore has the capability of handling substantially greater temperature variations in the scene. However, if the scene does not change, there is no A/C component to the signal itself Therefore A/C-coupled systems employ a mechanical chopper, with the attendant issues of weight, power, life time and delicacy as to mechanical shock.
A DC coupled thermal imaging system measures a tiny signal on top of a large DC background signal, which is a primary cause for noise limitation in the minimum discernable signal. The DC-coupled thermal imaging system must handle the relatively very large offset as well as the signal of interest. This complicates the system because the offsets differ from pixel to pixel, and the differences vary slowly with time, increasing spatial noise in the system. As a result, DC-coupled systems also employ a mechanical shutter to periodically re-calibrate the scene and minimize spatial noise. Both A/C and DC-coupled systems perform a similar comparative function by periodically shuttering the system at some time interval from a few seconds to a few minutes, usually resulting in an interruptive image freeze upon shuttering.
As described above in detail, the prior art micro-cantilever radiation sensors suffer from a number of disadvantages including:
(a) Because the temperature of the sensor element must reach a stable level before measurement may be made, the thermal time constant of the micro-cantilever must be made a fraction of the frame time. As a result the thermal conductance of the thermal insulator leg must be large, the temperature rise for a given irradiance is less and the bend of the responsive bi-material is decreased.
(b) Because in the past the use of aluminum or gold for the expansive material and silicon or silicon nitride for the less-expansive material, the difference in thermal expansion has been limited and the bend of the bi-material is decreased.
(c) Because the mechanical resonance of the micro-cantilever must be higher than ambient acoustic frequencies, the bi-material leg must be made stiff, and the bend of the bi-material is in response to irradiance is decreased.
(d) In past micro-cantilever bolometers the measure of irradiance is an analog signal, with the attendant 1/f, shot, Johnson, and amplifier noise, which together increase the irradiance required to exceed the noise of the sensor.
(e) As a result of the noise and insensitivity, bolometers have required the longest exposure possible to reach a stable temperature for measurement within the limited frame time, and thus have a limited ability to operate at a higher frame rate.
(f) Because of the need for stabilization of the substrate temperature, use of a costly and power-draining thermoelectric temperature regulator has been common.
(g) The linear measurements of past micro-cantilever bolometers are typically exhibited on an 8-bit gray scale display, limiting their linearly displayable upper irradiance to 256 times the least resolved display increment.
Thus, there is a need for an imaging sensor which operates by measuring absorbed power as heat regardless of the wavelength of the irradiance to provide a thermal-type irradiance imaging array, having improved range in target radiance and decreased sensor noise, and not requiring the use of thermoelectric temperature stabilization.